The present invention relates to a process for producing brass-coated steel wire for tire cord having improved drawability and substantially continuous crack-free coating after drawing to the final wire thickness. This invention also relates to a steel wire product having improved aged adhesion characteristics in certain rubber formulations.
The manufacture of steel wire for tire cord presently involves brass-plating the wire at an intermediate process size to assist in drafting the wire to finished size. The coating also serves to protect the base metal from corrosion and provide the necessary cord-to-rubber adhesion properties. The composition and thickness of the brass coating applied, as well as its continuity after the wire is drawn to finish size, all contribute to the performance of the wire in the stranded cord and to meeting each tire manufacturer's quality requirements. These requirements include a high level of both initial and aged adhesion of the cord in the particular rubber formulation of each tire manufacturer.
The various specifications from the major tire manufacturers require the weight of the coating as applied to be within a range of from 3.0 to 6.5 grams per kilogram of wire, and the composition of the coating to be within the range of from 62 to 70% copper and 30 to 38% zinc. Attempts have been made to improve the adhesion under aged conditions by slightly decreasing the copper percentage in the coating as well as decreasing the coating thickness. However, decreasing the copper content is known to cause the wire to have poorer drawing properties. This in turn causes poorer coating continuity at final thickness and decreases corrosion resistance of the wire. It also shortens the life of dies used in drawing the wire. To counter this last-mentioned effect, it often becomes necessary to use expensive diamond dies in order to achieve reasonable production rates.
Prior work described in French Patent No. 1,174,055 indicates the desirability of heating wire after it is coated first with zinc and then copper in separate layers. The heat treatment causes diffusion in the layers so as to form a brass coating which is stated to be more suitable for drawing and results in good rubber-to-wire adhesion. However, this work teaches that the final coating composition must be in the alpha-brass range, i.e. preferably containing 64 to 75% copper and the balance zinc. The patent also teaches that heating at higher temperatures will drive off any excess zinc present and produce the desired alpha-brass coating composition. The patent states that this benefit cannot be obtained when "coplated" brass coatings are heated. The reference indicates that the only benefit to be gained by heating "coplated" brass-coated wire is for rehabilitation of such wire in the event that it has poor drawing properties even though the composition is within the desired alpha-brass composition range. There is no suggestion in the reference that wire coplated with brass, having a copper content lower than the alpha brass range, might be heat-treated in order to produce superior improved drawability and superior aged adhesion properties, nor that such treatment would provide a coating which would be substantially continuous and crack-free after the wire is drawn to finished size.
U.S. Pat. No. 3,749,558, Dillenschneider discloses steel tire wire coated with nickel, copper and zinc in separate layers. The wire is then heated at 450.degree. C. for a few seconds to form a brass coating containing 70% copper. Also of interest is U.S. Pat. No. 2,563,113, Hindin et al which discloses steel tire cord wire plated with brass containing from 55-75% copper. This reference does not teach heat treatment of the coated wire. U.S. Pat. No. 3,961,740, Nakamoto et al discloses tire cord wire having a ternary coating containing 59-73% copper, 23-34% zinc, and 2-13% tin. U.S. Pat. No. 2,002,261, Domm discloses steel wire for tire bead applications, i.e. the wire is not cold-reduced after coating. The coating disclosed in this latter reference is a zinc layer weighing from 8 to 24 grams per kilogram of wire followed by a copper layer weighing 0.5 to 1.3 grams per kilogram of wire. The reference teaches that the layers diffuse upon vulcanization of the rubber article in which the wire is embedded so as to form an alloy coating. The proportion of zinc-to-copper in the layers is stated to be from 6 to 30 parts zinc for every one part of copper.
It is an object of the present invention to provide a process for treating brass-coated steel wire so as to significantly improve the drawability of the wire and provide a substantially continuous crack-free coating at final thickness.
It is another object of the invention to provide a brass-coated steel wire product made up of wire in stranded form having significantly improved aged adhesion properties in certain rubber formulations.
It is still another object of the invention to provide a process for producing steel tire cord wire by applying a coating having a composition within the range in which both alpha and beta phases are present under equilibrium conditions, and then heat-treating the wire to reduce the hardness of the coating to Rockwell B40-70 prior to drawing the wire to final thickness.